Replies to This Discussion

DeveloperPermalink Reply by Ryan Beall on December 20, 2010 at 1:53pm

The pitch stability is based on the CG being in front of the aerodynamic center.  Just because the tail is a "lifting tail" vice a negative lifting tail doesn't actually move the aerodynamic center.  It just makes the negative pitching moment that much harder to balance.  You can balance that moment by either extra elevator deflection or by moving the CG aft.  Moving the CG aft is making it more efficient for sure but still at the cost of Pitch stiffness. 

 

This is definition of the F/A18 A/B vs the F/A 18 E/F  Look at pics of the Super hornet landing.  It it tail heavy to the point of requiring conventionally opposite elevator control deflection.  This makes it clearly unstable but the AFCS computer is flying, not the pilot. 

 

So to answer your question....  Yeah move the CG aft to carry more of the load but know that the pitch stiffness will still decrease. 

 

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Permalink Reply by Paul G. on December 20, 2010 at 2:12pm

Moving the CG will neither increase nor decrease the aircraft's lifting capacity, it may make the airplane more pleasant to fly. The only things that can affect its steady state lifting capacity are 1) motor power, 2) airfoil shape(s), 3) lifting area and to a tiny extent 4) tail vs wing incidence (by affecting parasitic drag).

 

You can fly with a CG further back than normal however you will want to be careful. The easy way to test it is first make sure all the incidences are correct - this is important, use an hanger 9 angle pro or something similar. Then check that the airplane trim does not change much with airspeed - ie no elevator trim change from say 0% to 75% throttle, and no trim change from level flight to say a 45 degree dive. Once that is all correct check the actual elevator trim, ie look at the elevator surface, if its not flat then you can move the cg to improve the aerodynamics. Move it only a little at a time and check nothing else changes, ie the test's listed above are still ok. There will be some interaction with the above changes, for example changing the motor thrust line will change / mask changes in elevator trim - its an iterative process.

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Permalink Reply by William on December 20, 2010 at 2:34pm

Man I cant believe the results of this simple inquiry here at DIY Drones.

 

Thanks guys!  I really mean it.

 

My mind is like a sponge thanks for the drops O'water!

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DeveloperPermalink Reply by Ryan Beall on December 20, 2010 at 5:04pm

Moving the CG does affect the aircraft's lifting capacity.  To achieve steady level flight the forces and moments have to be equal to zero.  If the CG is creating a moement that requires the tail to produce a negative lifting force then that is nothing but detrimental to the total lift.  Hence the reason why canards were so popular (both are positive lifting but still satisfy the static stability problem!)   This is the reason the F/A 18 E/F has so much more "bring-back" aka can carry more weight back to the boat.  However there are a few more elements to the super hornet such as wing area etc that play into the equation here, but the aft CG is actually plays a large role.

 

 

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DeveloperPermalink Reply by Ryan Beall on December 20, 2010 at 5:18pm

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Permalink Reply by kdmasjj.stewart on February 1, 2011 at 11:39am

A "TAIL" heavy plane is harder to control. Most R/C planes have a CG range. I suggest to install what you want then "CG" the plane according to MFG spec.

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Permalink Reply by Curt Olson on February 2, 2011 at 8:36am

From my understanding, the main point of the lifting tail on the telemaster is to try to reduce the amount of pitch trim change needed between slow flight and fast flight. Typical trainers are very pitch stable (generally a good thing for trainers) and as speed increases, really want to pitch up. My Kadet Senior will loop if I trim it out for level slow flight and then apply full throttle (it may be a little higher powered than the manual recommends.) :-)
The idea of a lifting tail is that as speed increases, the lift of the tail also increases and tends to counter act the tendency of the plane to pitch up. The goal is to have an aircraft that doesn't require as much trim change between slow and fast speeds. You still need some increasing pitch moment when speed increase or you'll have an airplane that wants to dive steeper and faster and steeper and faster ...

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Permalink Reply by Mike Cowan on February 2, 2011 at 12:19pm

The replies by Beal and others describe how the cg position interacts with stability. A follow-on to the comment that the F/A-!8E/F landing with stabilator down deflection under control of the EFCS: A basic Ardupilot has the potential capability of stabilizing an unstable pitch loop(without increasing the max load). A knowledge of the pitch moment of inertia and other control loop parameters is required.

Increasing power and angle of attack increases payload with a penalty of reducing the maximum g the airframe can sustain under maneuvers. Again an Ardupilot g-limiting control law could be written to keep the plane in a suitable g envelope. For F/A-18 fans, watch the rudders on take off.  They are each rotated to full inboard deflection, because the stabilator exerts insufficent aero moment to raise the nose on carrier take-off.  The pilot grabs the two grab bars, the EFCS goes off the cat with full control of the stabilator and rudder fade-out. The pilot does not take control until normal climbout is established.

 

Some DIYer out there may have a wind tunnel under construction to build interesting and useful unstable aircraft.  Technology change is exponential.

 

Mike Cowan

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Permalink Reply by Andrew Rabbitt on February 3, 2011 at 12:09am

A simple open-loop elevator deflection-v-IAS characteristic added as an offset to the elevator PWM servo output could be enough to allow a rearward CoG whilst yielding enough pitch stability to be flyable.  If the elevator is in the propwash, it may need to be mapped against IAS and propellor speed/RPM.

Not failsafe, mind...

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